A 3D neutron kinetic code DiFis is designed for steady-state and transition calculation for cores with hexagonal fuel assemblies. The code uses traditional time-dependent diffusion equations in a two-group approximation and takes into account six delayed group neutron precursors. A variation finite element method is used for solution of diffusion equations. The thermal-hydraulic model of the code is based on an “average” fuel rod model. Each fuel assembly contains such a rod, which provides heat transfer from a fuel pellet through the gas gap and the cladding to the coolant. The thermal-hydraulic model is used for feedback correction of macroscopic nuclear cross-sections. The code DiFis has been used for the AER-DYN-002 benchmark problem, which describes an eccentric control rod ejection at hot zero power condition in a VVER-440 type reactor. The only feedback mechanism taken into account is a Doppler one. Comparison with well-known nodal kinetic codes shows acceptable accuracy of DiFis. As an example of transition calculation, a VVER-1000 core behavior during fast power reduction is described.

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